NAME

       SDL::Tutorial::Animation - Creating animations with SDL

   CATEGORY
       Tutorials

SYNOPSIS

               # to read this tutorial
               $ perldoc SDL::Tutorial::Animation

               # to create a demo animation program based on this tutorial
               $ perl -MSDL::Tutorial::Animation=sdl_anim.pl -e 1

ANIMATING A RECTANGLE

       Now that you can display a rectangle on the screen, the next step is to animate that
       rectangle.  As with movies, there's no actual motion.  Computer animations are just very
       very fast slideshows.  The hard work is creating nearly identical images in every slide
       (or frame, in graphics terms).

       Okay, it's not that difficult.

       There is one small difficulty to address, however.  Once you blit one surface onto
       another, the destination is changed permanently.  There's no concept of layers here unless
       you write it yourself.  If you fail to take this into account (and just about everyone
       does at first), you'll end up with blurry graphics moving around on the screen.

       There are two approaches to solve this problem, redrawing the screen on every frame and
       saving and restoring the background for every object drawn.

   Redrawing the Screen
       Since you have to draw the screen in the right order once to start with it's pretty easy
       to make this into a loop and redraw things in the right order for every frame.  Given a
       SDLx::App object $app, a SDL::Rect $rect, and a SDL::Color $color, you only have to create
       a new SDL::Rect $bg, representing the whole of the background surface and a new mapped
       color $bg_color, representing the background color.  The colors need to be mapped to the
       format of the current display. This is done by SDL::Video::map_RGB.

       my $color = SDL::Video::map_RGB (       $app->format,       $rect_r,       $rect_g,
       $rect_b, );

       my $bg_color = SDL::Video::map_RGB (       $app->format,
             $bg_r,       $bg_g,       $bg_b, );

       You can write a "draw_frame()" function as follows:

               sub draw_frame
               {
                       my ($app, %args) = @_;

                       SDL::Video::fill_rect($app,  $args{bg},   $args{bg_color}   );
                       SDL::Video::fill_rect($app, $args{rect}, $args{rect_color} );
                       SDL::Video::update_rects($app, $args{bg} );
               }

       Since you can change the "x" and "y" coordinates of a rect with the "x()" and "y()"
       methods, you can move a rectangle across the screen with a loop like this:

               for my $x (0 .. 640)
               {
                       $rect->x( $x );
                       draw_frame( $app,
                               bg   => $bg,   bg_color   => $bg_color,
                               rect => $rect, rect_color => $color,
                       );
               }

       If $rect's starting y position is 190 and its height and width are 100, the rectangle (er,
       square) will move across the middle of the screen.

       Provided you can keep track of the proper order in which to redraw rectangles and provided
       you don't need the optimal speed necessary (since blitting every object takes more work
       than just blitting the portions you need), this works quite well.

   Undrawing the Updated Rectangle
       If you need more speed or want to make a different complexity tradeoff, you can take a
       snapshot of the destination rectangle before you blit onto it.  That way, when you need to
       redraw, you can blit the old snapshot back before blitting to the new position.

       Note:  I have no idea how this will work in the face of alpha blending, which, admittedly,
       I haven't even mentioned yet.  If you don't know what this means, forget it.  If you do
       know what this means and know why I'm waving my hands here, feel free to explain what
       should and what does happen and why.  :)

       With this technique, the frame-drawing subroutine has to be a little more complicated.
       Instead of the background rect, it needs a rect for the previous position.  It also needs
       to do two updates (or must perform some scary math to figure out the rectangle of the
       correct size to "update()".  No thanks!).

               sub undraw_redraw_rect
               {
                       my ($app, %args) = @_;

                       SDL::Video::fill_rect($app, $args{old_rect}, $args{bg_color}   );
                       SDL::Video::fill_rect($app,  $args{rect},     $args{rect_color} );
                       SDL::Video::update_rects($app, $args{old_rect} );
                       SDL::Video::update_rects($app, $args{rect} );
               }

       We'll need to create a new SDL::Rect, $old_rect, that is a duplicate of $rect, at the same
       position at first.  You should already know how to do this.

       As before, the loop to call "undraw_redraw_rect()" would look something like:

               for my $x (0 .. 640)
               {
                       $rect->x( $x );

                       undraw_redraw_rect( $app,
                               rect       => $rect,  old_rect => $old_rect,
                               rect_color => $color, bg_color => $bgcolor,
                       );

                       $old_rect->x( $x );
               }

       If you run this code, you'll probably notice that it's tremendously faster than the
       previous version.  It may be too fast, where the alternate technique was just fast enough.
       There are a couple of good ways to set a fixed animation speed regardless of the speed of
       the processor and graphics hardware (provided they're good enough, which is increasingly
       often the case), and we'll get to them soon.

SEE ALSO

       SDL::Tutorial::Drawing
           basic drawing with SDL Perl

       SDL::Tutorial::Images
           animating images

AUTHOR

       chromatic, <chromatic@wgz.org>

       Written for and maintained by the Perl SDL project, <http://sdl.perl.org/>.  See "AUTHORS"
       in SDL.

BUGS

       No known bugs.

COPYRIGHT

       Copyright (c) 2003 - 2004, chromatic.  All rights reserved.  This module is distributed
       under the same terms as Perl itself, in the hope that it is useful but certainly under no
       guarantee.